I have updated my Mac to macOS Sierra. Since then, when I build my projects with Packages, the builds fail. Am I cursed?

A

No, you are not cursed. At least, as far as I know. It's a compatibility issue between Packages and macOS Sierra.

There are actually 2 problems:

Packages uses deprecated security APIs to sign the distributions and raw packages as it needs to work on Mac OS X 10.5 and later. And since macOS Sierra, at least one of those deprecated APIs is now broken. Hence the build failure.